This application is mandated by the RFA for U01, which will allow us to synergize with other outstanding mucosal immunologists and cooperatively uncover and harness the neglected mucosal protective functions of IgD, an evolutionarily conserved yet mysterious class of antibody. Human upper respiratory mucosa contains abundant IgD-producing B cells that release IgD into nasal, lachrymal, salivary, mammary and bronchial secretions. Numerous clinical and immunological observations of IgD in the past 50 years strongly suggest that IgD has important functions in respiratory mucosal immune defense. The broad, long-term objectives of this project are to elucidate the regulation of IgD production as well as the mechanism and function of IgD in respiratory immunity in order to develop vaccines that harness the functions of IgD to combat respiratory infections and therapies to treat immunopathologies associated with IgD hyperproduction. This study hypothesizes that B cells from upper respiratory mucosa undergo IgD production and diversification controlled by vitamin D3 and that IgD produced reacts against respiratory pathogens and contributes to mucosal immunity by activating local and systemic innate immune cells such as basophils and mast cells through interaction with heparin, eosinophil cationic protein and inflammasomes. Three aims are proposed. Aim 1: To elucidate the regulation of IgD class switching and production in the respiratory mucosa. Aim 2: To determine the reactivity, clonal diversity, evolution and protective function of mucosal IgD. Aim 3: To dissect the mechanisms by which IgD activates mucosal and systemic immune responses. This study will take advantage of cells and tissues from healthy donors and patients with hyper-lgD syndrome, an autoinflammatory periodic fever syndrome, as a disease model. The regulation of IgD production by vitamin D3 and the mechanism by which IgD triggers immune activation will be investigated by biochemical, molecular and cellular biology techniques. The reactivity and protective functions of mucosal and systemic IgD responses upon respiratory pathogen exposure and after vaccination will by studied by single B cell sorting, cloning and expression, high-throughput next-generation sequencing and in vitro cell culture-based methods. RELEVANCE: This project will establish clinically relevant information on IgD responses after vaccination and lead to the isolation of protective IgD clones that may be used clinically to combat respiratory infections. It will also support the use of vitamin D modifying regimens in treating immunopathologies and establish that more effective vaccination strategies against respiratory pathogens should actively boost IgD responses.